This invention relates broadly to an assembly providing a moisture barrier on a lower surface of a wood casing, of the type used for window frames, door jambs, moldings for external building siding such as brick, stone, block, wood or vinyl, and other types of casings in order to prevent wood rot.
Such casings have bases which are often subjected to moisture or standing liquid such as rain water at various times throughout their use service lives. As a result, rotting of such casings at their bases is an ongoing problem. The problem is enhanced by the fact that the end grain of such a wood casing on its exposed lower surface has an affinity for absorbing moisture from the surface on which it is supported. The usual handling of these casings during transportation from mill to dealer, from dealer to job site and, thereafter, from handling at the job site can cause scratching, scoring, gouging or other disruptions of the end grain which will enhance moisture absorption into the end grain of wood.
Previous efforts to prevent wood rot of wood casings have not been a success. Attempts have included copper plating of fastener members, wrapping of preservative packages, surrounding the wood with moisture impervious materials, and the like. These prior attempts have not recognized that the source of wood rot is to a large extent based upon water absorption through the exposed end grain. Thus, because this point was not appreciated, prior efforts have been flawed from a design point, and have increased the problem. Exposed major surfaces do not usually contribute to wood rot, because there is little moisture to penetration. The moisture content must approach 25% for wood rot to be initiated, and that content cannot normally be approached unless an end grain surface is available. The end grain surface allows moisture to wick into the wood, with the result that the high moisture content may be achieved, particularly if left in standing water.
Placing the end grain surface on concrete, such as frequently occurs with slab construction, will not solve the problem. Concrete is a relatively porous material, so moisture may actually migrate through the concrete to the end grain surface. Thus a dam, plastic barrier, or the like applied to the major surfaces will not solve the problem, where the end grain surface rests upon concrete.
By means of my invention, this and other difficulties encountered using wood casings of the prior art are substantially reduced, if not altogether eliminated.